1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine and, particularly, to a control system having a function of determining deterioration of an oxygen concentration sensor provided in the exhaust system of the engine.
2. Description of the Related Art
Japanese Patent Laid-open No. H04-36651 (JP-'651) discloses a method of adjusting a control gain of an air-fuel ratio feedback control according to an output of an oxygen concentration sensor when the oxygen concentration sensor is determined to have deteriorated. In this method, the deterioration of the oxygen concentration sensor is determined based on the output of the oxygen concentration sensor at the beginning of an interrupting operation of the fuel supply of the internal combustion engine, or at the restart of the fuel supply.
Specifically, the control gain of the air-fuel ratio feedback control is adjusted so that the control gain decreases as the degree of deterioration of the oxygen concentration sensor increases.
In air-fuel ratio feedback control, if the oxygen concentration sensor is not deteriorated when the target air-fuel ratio is made to change as shown by the thin solid line L1 of FIG. 21A, the sensor detected air-fuel ratio AFS, which is calculated based on the sensor output, changes as shown by the thick solid line L2 of FIG. 21A. An air-fuel ratio control signal SCTL, which is generated by the PID control based on the sensor detected air-fuel ratio AFS, changes as shown by the dashed line L3 of FIG. 21A.
If the oxygen concentration sensor becomes deteriorated, the sensor detected air-fuel ratio AFS changes as shown by the solid line L2 of FIG. 21B, and the air-fuel ratio control signal SCTL greatly overshoots as shown by the dashed line L3 of FIG. 21B. In this state, if the control gain is adjusted as described above, the sensor detected air-fuel ratio AFS and the air-fuel ratio control signal SCTL respectively change as shown by the solid line L2 and the dashed line L3 of FIG. 21C. That is, the overshoot of the air-fuel ratio control signal SCTL is eliminated but, as shown by the dashed line L3 of FIG. 21A, the air-fuel ratio control signal SCTL, which corresponds to the characteristic obtained when the oxygen concentration sensor is normal, is not obtained. In the example shown by FIGS. 21B and 21C, the oxygen concentration sensor has deteriorated beyond a certain amount. Therefore, it is considered that the sensor detected air-fuel ratio AFS does not indicate the actual air-fuel ratio and that the air-fuel ratio control signal SCTL indicates an air-fuel ratio changing characteristic which is similar to changes in the actual air-fuel ratio (refer to FIG. 21A).